Signaling system



Jan. 22, 1935.

M. ARTZT 1,988,472

SIGNALING SYSTEM Filed NOV. 20, 1931 Fig.1.

26\ g AMPLIFIER 28 RADIO 22 TRANSMITTER R Q. E 8 L F'|g.2.. 3 Q E 42 lo I INVENTOR.

ly T- weak HIS ATTORNE Maurice Artz't,

Patented Jan. 22, 1935 UNITED STATES PATENT OFFICE.

SIGNALING SYSTEM Maurice Artzt, Camden, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 20, 1931, Serial No. 576,266

6 Claims. (Cl. 179-171) My invention relates to signaling systems and, of these tubes being connected in parallel, the more particularly, to systems of the type wherein j tments being such that the voltage waves the transmitted signals are representative of light in the tWO Plate Circuits are f eq a amplitude impinging upon photdelectric 11, but one hundred and eighty degrees out of phase Heretofore, in facsimile transmission systems When the circuit i ba a c d, t W ve to b 5,

wherein a photocell is used to control the developmodulated being pp d t the Screen-grid of merit of signals in accordance with conditions each tube, the Control d of One tube hav n s of light and shade to which it is exposed, the biasing potential varied in accordance with conmodulation of the carrier wave has been effected ditiohs light and shade to which e p Otee ll 10 through the use of alight chopper and an assois subjected, and wherein the potential of the 10 ciated alternating current amplifier, or a direct Control d of the other tube is maintained current amplifier and modulator. In the vasubstantially constant p n n y f omrious systems embodying a light chopper, the ring variations in conditions of light and shade wave shape developed has not been entirely satisto w h the photocell is subjected.

l5 factory. That is, the Wave has not been of such My invention resides in the Provision of appa- 5 shape as t prgvide for t advantageous ratus and system of the character hereinafter ditions of operation. This disadvantage is atdescribed and l im tri t t t light chopper, For the purpose of illustrating myinventior an Furthermore, in many of the facsimile scanembodiment thereof is SheWn in the -W ners proposed heretofore, it has been quite awkw rein 20 ward and difilcult to place the chopper in the Figure 1 is a diagr a c V Of a System mechanical system without sacrificing. certain embodying y nv n ion;

desirable features. In the various systems em- Fig. 2 is a graphical il ust at in explanation bodying a direct current amplifier, difficulties of the principle of operat n o the ystem in have been encountered due to a pronounced nd l 25 tendency of the amplifier to drift, that is, to Figs? 3 e 4 are fragmentary, d ag a matic depart from its adjustment in such degree as to V ews, illustrating sev al m dific in materially interfere with the fidelity of the repro- With reference to Fig. 1, my improved appaduced pictu1e I ratus comprises a balanced circuit including a With the foregoing in mind, it is one of the P of Screen-grid tubes, wherein t e Plate Cir- 30 objects of my i nti t provide improved apcults of the latter are connected in parallel and paratus for modulating an electrical wave which wherein e electrical Wave to be modulated is avoids the various disadvantages referred to in pp d across a resistor also included in the balapparatus of this character proposed heretofore eneed Circuit and having ts e ds Connected reand which, more specifically, is relatively stable pe v to the ids of the tubes, the 35 and capable of holding its setting over long adjustment bein such that the volta e imp essed periods of time, as well as simple to build and use. on the screen-grids a e the sam a y t t Other objects and advantages will hereinafter but one hu dr d and i hty de s ut of p as appegr, The biasing potential on the control grid of one In accordance with my invention, an electrical tube is varied in accordancewith occurring vari- 40 wave is mcdmated by employing b l d i ations in an electrical condition to unbalance cuit which includes thermionic tubes of the sothe circuit The potential on the cont l rid called screen-grid type, and wherein the wave of the other tube is maintained substantially conto be modulated is applied to the screen-grid oi stant independently of t occu variations each tube, the plate circuits of the latter being in he e ect cal C ndition referred to. 45 connected in parallel, and the adjustments being More p c fi y, my improved pp atus'comsuch that the voltage waves in the two plate cirp for p a balanced circuit including cults are of equal amplitude but one hundred, a resistor 10 across which the electrical wave to and eighty degrees out of phase when the circuit be modulated is applied by connections 11 and is balanced. 12. A pair of similar screen-grid tubes 13 and 50 More particularly, in accordance with my in- 14 are connected in and form a part of the balvention, apparatus for modulating an electrical anced circuit, the plates of these tubes being conwave is provided wherein a pair of thermionic nected to the common point 15, whereby the plate tubes of the so-called screen-grid type are concircuits are connected in paralleL- The screen- 65 nected in a balanced circuit, the plate circuits grids 16 and 1'7 of these tubes are connected, re-

spectively, to the ends of the resistor 10, as shown.

The plates of the tubes 13 and 14 are connected to the resistor 10 through. a common connection including a resistor 18, a battery 19, and an adjustable contact 20. In operation, the contact 20 is initially adjusted so that the voltage waves impressed on the screen-grids 16 and 17 are one hundred and eighty degrees out of phase at any instant, and of such relative amplitudes that the voltage amplitudes in the plate circuits are equal and opposite. The bias for the control grid 21 of the tube 14 is determined by adjustment of a contact 22 associated with a resistance 23 connected, as shown, across a battery 24. The control grid 25 of the tube 13 is also biased by the battery 24, through a suitable resistor 26.

In using my improved apparatus and controlling development of picture signals in a facsimile transmission system, a photocell 27 is connected, as shown, across the resistor 26 through a battery, 28 supplying a suitable positive potential to the anode of the photocell. As is well known, the photocell 27 draws more or less current depending upon conditions of light and shade to which the same is subjected. In the system shown, variations in these conditions are determined by the amount of light reflected from the surface of a picture wrapped about the usual drum 29 forming part of the transmitter.

In setting up the apparatus, the contact 22 is so adjusted that the bias on the control grid 21 is the same as that on the control grid 25 under conditions whereat no light is received by the photocell 27, in which case the alternating current component in the resistor 18 is balanced to zero, because the voltage waves in the plate circuits of the tubes 13 and 14 are then of the same amplitude and one hundred and eighty degrees out of phase.

The adjustments referred to are such that the tubes. 13 and 14 operate on the straight line portions of their respective characteristic curves, and in the region A, as indicated in Fig. 2. It is proposed to make the resistor 18 relatively high, for example, 2 megohms, so that the screen-grid voltages, as determined by a battery 30 of about 6 volts, are very low compared to the voltage applied to the plates by the battery 30 and the battery 19 of about volts.

The alternating current carrier is supplied by a suitable thermionic oscillator designated generally by the reference numeral 31, the voltage being such that it will not swing the screen-grids 16 and 17 beyond the straight line portions of the characteristic curves.

In considering the operation of the system, it is important to note that, on account of the adiustments and the connections as shown in Fig. 1, both tubes 13 and 14 have such potentials at any instant on their respective and corresponding electrodes when the photocell is receiving no light, that the circuit remains balanced independently of occurring variations in the amplitude of the carrier wave. Further, the control grid bias of the tube 13 is equal, substantially, to the potential supplied by the battery 24, since the potential gradient across the resistor 26 is very low when the photocell 27 is receiving no light.

In operation, when light is reflected on the photocell 27, the same will draw a corresponding amount of current to cause variation in the voltchanged, so that the amount of alternating current in the plate circuit of this tube is no longer balanced out by the alternating current in the plate circuit of the tube 14. The difference appears as an alternating current voltage across the resistor 18, and this voltage is applied through the connections 32 to the grid circuit of a suitable amplifier 33. The modulated carrier is caused to pass through a suitable filter 34 designed to pass the carrier and its side bands but to reject those frequencies which are used to modulate the carrier. After passing through the filter 34, the modulated carrier is amplified by a suitable amplifier 35, and supplied to the usual radio transmitter 36.

A suitable oscillator, designated by the reference numeral 37, supplies to the transmitter a synchronizing .tone of any suitable frequency, such as, for example, 230 cycles.

With the adjustment Just described, the manner of operation is sometimes referred to as' positive modulation, this action being determined by the fact that the adjustments are such that the tubes 13 and 14 operate in the region A.

For the purpose of providing for the reverse operation, referred to as negative modulation, a second contact 38 is associated with the resistor 10 and adjusted at a point whereat the circuit will be balanced when the photocell 27 is subjected to the maximum light intensity. A switch 39 provides for convenient connection to either the contact 20 or the contact 38. With the contact 38 elfective, the tubes 13 and 14 operate in the region B, and the percentage of modulation of the carrier wave increases consequent and corresponding to conditions of decrease in light intensity to which the photocell is subjected.

In embodying my improved apparatus in systems requiring high power output, or systems wherein the conditions are such that resistance coupling, as shown in Fig. 1, would not lend itself particularly to the conditions of frequency, it is proposed to employimpedance or transformer coupling at the input and output ends of the balanced circuit. For example, in Fig. 3, the transformer 40 is shown substituted for the resistor 10 in Fig. 1, thereby providing for transformer coupling between the source of the carrier wave and the screen grids 16 and 17. The connections, otherwise, are the same as shown in Fig. 1.

When transformer coupling is used in this way, the connection between the battery 19 and the secondary winding 41 of the transformer is permanent and substantially at the middle point of this winding. With this connection, the balance is effected by adjustment of the resistance 23, and the selection for positive or negative modulation is made by providing a second contact 42 corresponding to, and serving the same purpose as the contact 38 in Fig. 1.

Fig. 4 shows the manner of connecting a choke coil 43 in substitution for the resistor 18 in Fig. 1 when it is desired to employ impedance coupling between the output of the balanced circuit and the grid circuit of the amplifier 33. i

From the foregoing it will be seen that an improved apparatus for modulating. an electrical wave has been provided which is relatively stable and is capable of holding its adjustment for long periods of time, which is relatively simple and 1,css,472

easy to operate, and which has the advantages referred to when embodied in a facsimile transmission system in place of the light choppers and direct current amplifiers and modulators used heretofore.

My apparatus may also be used to modulate an audio frequency for line transmission of facsimile signals, it may be used directly at a radio transmitter for modulating the carrier, and, also, it may be used in television systems to eliminate the audio system, either by supplying a 200 kilocycle or 300 kilocycle carrier to the modulator, or by modulating the radio carrier directly.

Furthermore, it is contemplated to embody my improved modulating apparatus in a system wherein other-means than light-controlled apparatus, such as a facsimile transmitter and the associated photocell, are employed for modulating the carrier wave by varying the potential across the resistor 26. For example, in an audio system, the photocell 27 in Fig. 1 would be replaced by a condenser microphone or other suitable device for translating sound waves into electrical waves. In such case, the adjustments for positive modulation would be such that the circuit is balanced at conditions whereat the microphone is not being subjected to any sound waves, or conversely, for negative modulation the circuit would be balanced when the microphone is subjected to the peak sound wave.

I claim as my invention:

1. In apparatus for modulating an electrical wave, a balanced circuit, a resistor forming part of said circuit and through whichsaid wave is caused to pass, a second resistor forming part of said circuit and across which the modulated wave appears, a screen-grid tube connected to an end of said second resistor and having its screento said end of said second resistor and having its screen-grid connected to the other end of said first-named resistor.

2. In apparatus for modulating an alternating current wave in accordance with variations in an electrical efiect, first and second thermionic tubes each having a plate and a control grid and a screen-grid, the plate circuits of said tubes connected in parallel, a resistor having its ends connected respectively to the screen-grids of said tubes, connections for applying said wave across said resistor, means for supplying the control grid of one of said tubes with a biasing voltage, means associated with said first-named means and operable to efiect variations in said voltage corresponding to occurring variations in said electrical efiect, and means for supplying to the control grid of the other of said tubes a biasing voltage maintained substantially constant independently of occurring variations in said electrical eiiect. I

3. In apparatus for modulating an alternating current wave in accordance with variations in an electrical effect, a balanced circuit, screen-grid tubes forming part of said circuit, a resistor connected between the screen-grids of said tubes, means for applying said wave. across said resistor, and a photosensitive device connected to only one of said tubes.

4. In a system for developing electrical signals corresponding to variations in an electrical effect, thermionic tubes of the screen-grid type and each provided with a control grid, means for applying an electrical wave to the screen-grids of said tubes, and direct-current connection for impressing on the control grid of only one of said tubes a biasing direct-currentpotential varying in accordance with occurring variations in said effect.

5. In a system for developing electrical signals corresponding to variations in an electrical efiect, a balanced circuit, thermionic tubes of the screengrid type each provided with a plate and a control grid, said tubes forming part of said circuit and having their plates connected to a common point therein, a first resistor forming part of said circuit and having its ends connected respectively to the screen-grids of said tubes, means for applying an alternating current carrier wave across said resistor, a source of direct current and a second resistor connected in series therewith, such series being connected between said common point and a point on said first resistor determinstant independently of occurring variations in said electrical efiect from said given value.

6. In apparatus for modulating an alternating current wave in accordance with variations in an electrical effect, first and second thermionic tubes each havinga plate and a control grid and a screen-grid, the plate circuits of said tubes connected'in parallel, connections for applying said ":wave to the screen-grids of said tubes, means for supplying the control grid of one of said tubes with a biasing voltage, means associated with said first-named means and operable to eifect variations in said voltage corresponding to occurring variations in said electrical efiect, means including a resistance providing a direct-current connection between said second-named means and said control grid, and means for supplying to the control grid of the other of said tubes a biasing voltage maintained substantially constant independently of occurring variations in said electrical efiect.

MAURICE AR'I'ZT. 

